1. Field of the Invention
The present invention relates to a sheet feeder which feeds one sheet at a time by separating a stack of sheets loaded on a paper feed tray one by one, as well as to an image forming apparatus having the sheet feeder.
2. Description of the Related Art
Conventionally, in an image forming apparatus, such as a copier or a printer, single-cut sheets which can feed continually are usually limited to bond paper or plain paper designated by the manufacturer of the copier. Since such paper has a low degree of surface smoothness, there is no risk of feeding doubled or overlapped sheets because of absorption between sheets when sheets are withdrawn one at a time from the loaded bundle of paper. On the other hand, in association with recent diversification of a recording medium, there is a growing demand for formation of an image on a sheet having a smooth surface, such as art paper or coat paper which is coated with a view to giving paper a high gloss and brightness and to responding to market demands for color printing, as well as on paper for use with an over-head projector, cardboard, or tracing paper. Such paper, i.e., paper for use with an over-head projector, tracing paper, art paper, or coat paper, has a high degree of surface smoothness. In a case where such paper is loaded at high humidity, sheets of paper attach together, the existing image forming apparatus suffers several problems such as feeding of overlapped sheets or feeding errors.
To solve the foregoing problems, there has already been proposed, for example, a method which uses a paper feeder 1 shown in FIG. 24 and which is described in Japanese Patent Application Laid-open No. Sho-62-249835. Under this method, air is blown by an air blower 7 to the leading side end of the stack of sheets 5 loaded on a paper feed table 3 from a direction parallel to the upper surface of the sheets 5. At the same time, a negative pressure is applied from an unillustrated negative pressure generator to an air supply cylinder 9 having an opening located at a position above the sheet 5, thus drawing external air in the vicinity of the opening 11 into the air supply cylinder 9. In this paper feeder 1, the topmost paper of the sheets 5 loaded on the paper feed table 3 is slightly floated in the air by drawing action through the opening 11, and air is blown from the air blower 7 toward a clearance formed between the topmost sheet 5 and the second-top sheet 5, thus ensuring separation of sheets.
There has also been proposed another method which uses a paper feeder 13 shown in FIGS. 25 to 27 and which is described in Japanese Patent Application Laid-open No. Hei-4-23747. Under this method, air blowing means 15 blows air toward the sheets 5 from a supply opening 19 formed opposite the side edges of the sheets 5, thus floating up the sheet 5. In cooperation with a-rolled, brush-shaped, or plate-shaped sheet press member 17 in contact with the upper surface of the sheet 5, air is introduced to the space between the sheets, thus separating the sheets 5 one by one. If there is not the sheet press member 17, a greater number of sheets than are required are floated into the air, so that a few sheets in the vicinity of the topmost sheet become insufficiently separated. However, the paper feeder 13 is capable of preventing the sheets 5 from being excessively floated into the air, as well as of introducing air into the space between the sheets, thus weakening attracting force acting between sheets and ensuring separation of sheets.
Still another type of paper feeder is described in Japanese Utility Model Application Laid-open No. Sho-61-21735, wherein a ventilation port is formed in the side surface of a paper feed tray, as well as being connected to an air blower through a pipe. Yet another type of paper feeder is described in Japanese Patent Application Laid-open No. Hei-3-211136, wherein an air flow to be blown toward the side surface of sheets is changed.
However, for the purpose of separating sheets by blowing air toward the side surface of the leading end of sheets and by drawing the topmost sheet upwardly, the paper feeder 1 of FIG. 24 must be provided with the air blower 7, the air supply cylinder 9, and a negative pressure generator, thereby rendering the paper feeder bulky, adding to the cost of products and running costs, and resulting in an increase in noise. Even the paper feeder having the air blower connected to the ventilation port through a pipe or the paper feeder which changes an air blow to be blown toward sheets becomes bulky and results in an increase in the cost of products.
In contrast, the paper feeder 13 of FIG. 25 that separates sheets by means of the air blowing means 15 and the sheet press member 17 is capable of manufacturing products at comparatively low expenses. However, in a case where paper is cut, thus producing large burrs which in turn close the spaces among sheets, or where the edges of the sheets are in a ragged state and become misaligned in a moving-away direction relative to an air supply opening, a small volume of air fails to enter the spaces among sheets. Consequently, the volume of air must be increased, thus adding to running costs and resulting in an increase in noise.
In the case of a paper feeder shown in FIG. 28A in which air blowing means 28 blows air toward the side surface of sheets loaded on a paper feed tray, thus causing a sheet to blow off and separating closely-attached sheets, if air is blown toward the side edges of the sheets at one time in a manner as shown in FIG. 28B, sheets positioned above a boundary surface 19 where sheets weakly attach to each other are caused to blow off in a stacked manner as shown in FIG. 28C. Consequently, it becomes impossible to let air through the spaces among the stack of sheets floating in the air, so that the sheets remain attached to one another. Eventually, the paper feeder causes feed errors or feeding of overlapped sheets.